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ODB and the ODMG(Object Data Management Group)

Object (Oriented) Databases (ODB)

The ODMG standard:

Object model

ODL

OQL

Language-bindings

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ODMG: The Object Model

• Object:– Objects have identity (OID) and state, OID

never changes, state may change over time– Objects may have names (entry points to the

Database)– Objects have a lifetime (either persistent or

transient

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ODMG: The Object Model

• Object:– Objects have structure (atomic or Collection)– The structure specifies how to create the object

using a type constructor• Atomic objects are any objects that are not

collections• They may be composite (have internal structure)• They are not tuples in the relational sense of the

word tuple

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Literals

• Have state, but no OID, state is never changed (”constant objects”)

• State may be atomic or complex:– Atomic literals are simple, predefined types– Structured literals (roughly records or structs)– Collection literals (collections like in the Java or

.NET API, but persistent)

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Standardinterfaces i ODMG

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Atomic (domain) Objects• Called atomic, but are often composite

(structured)

• Corresponds to entities in a conceptual model

• Objects have:– Properties (state):

• Attributes• Relationships

– Operations (behaviour)

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OBS! Inverse =>

automatisk integritet

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Inheritance

• Between interfaces:– only (abstract) behaviour (specification) – Any properties defined in an interface are not

inherited– “keyword”: ”:”

• Between classes:– both state and behaviour– Keyword: “extends

• Like Java or C#

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The Extension of a Class (extent)

• All objects belonging to a class is automatically stored persistently in the extension of the class

• The extension of a class is the set of all objects of that class

• The extension is declared using the keyword extent (not extends, which specifies inheritance)

• The extension of some class T is an object of type Set<T>

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Inheritance and Class Extension

• class B extends A

Means (among other things) that

• The extension of B is a subset of the extension of A

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ODL: Object Definition Language

• Graphical notation:

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Note! extends extent

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Inheritance between Interfaces

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Only operations are

inherited

Must be repeated

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OQL: Object Query Language

• Based on SQL– Non-procedural– Supports object navigation using references– Embedded or stand-alone

• For instance departments in an Engineering College:SELECT d.dnameFROM d IN departmentsWHERE d.college = ’Engineering’

• Returns a Bag of dname, SELECT DISTINCT returns a Set

Entry point is the extension of class

Department

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Path-Expressions

• departments; – //returns a reference to the extension of Department

• csdepartment;– //a reference to the CS-dept-object

• csdepartment.chair;– //a path to the chair object of the CS-dept-object (type Faculty)

• csdepartment.chair.rank;– //the rank attribute of the chair -object of the CS-dept-object

• csdepartment.has_faculty;• //returns Set<Faculty>

– etc. etc.

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Java-binding - FastObjectsDatabase db = new Database();Transaction txn = new Transaction();db.open("addressDB",Database.OPEN_READ_WRITE);txn.begin();OQLQuery query = new OQLQuery( "select x from Person x where x.name = \"Doug Barry\"");Collection result = (Collection) query.execute();Iterator iter = result.iterator();while ( iter.hasNext() ){ Person person = (Person) iter.next(); person.address.street = "13504 4th Avenue South"; }txn.commit();db.close();...

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try{ MyClass myObject = new MyClass(); db.bind(myObject, "myName"); //root } catch (ObjectNameNotUniqueException exc){ txn.abort(); System.out.println( exc); } try{ MyClass myObject = (MyClass)db.lookup("myName"); System.out.println(myObject); } catch(ObjectNameNotFoundException exc){ System.out.println(exc); txn.abort(); }

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Using Extension

//...

db.makePersistent( new MyClass() );

//…

// print all stored instances of MyClass

Extent myClasses = new Extent(MyClass.class);

while ( myClasses.hasNext() ) {

System.out.println( myClasses.next() );

}

//...

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Conclusion:

OOPL

+ embedded ”SQL”

+ persistent Collection

==

ODB

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